It is sought more particularly here below in this document to describe problems existing in the field of seismic data acquisition for oil prospecting industry. The invention of course is not limited to this particular field of application but is of interest for any technique that has to cope with closely related or similar issues and problems.
The operations of acquiring seismic data in the field conventionally use networks of seismic sensors, like accelerometers, geophones or hydrophones. In a context of seismic data acquisition in a marine environment, these sensors are distributed along cables in order to form linear acoustic antennas normally referred to as “streamers” or “seismic streamers”. The network of seismic streamers is towed by a seismic vessel.
The seismic method is based on analysis of reflected seismic waves. Thus, to collect geophysical data in a marine environment, one or more submerged seismic sources are activated in order to propagate seismic wave trains. The pressure wave generated by the seismic sources passes through the column of water and insonifies the different layers of the sea bed. Part of the seismic waves (i.e. acoustic signals) reflected are then detected by the sensors (e.g. hydrophones) distributed over the length of the seismic streamers. These acoustic signals are processed and retransmitted by telemetry from the seismic streamers to the operator station situated on the seismic vessel, where they are stored.
A well-known problem in this context is the positioning of the seismic streamers. Indeed, it is important to precisely locate the streamers in particular for:                monitoring the position of the sensors (hydrophones) in order to obtain a satisfactory precision of the image of the sea bed in the exploration zone; and        detecting the movements of the streamers with respect to one another (the streamers are often subjected to various external natural constrains of variable magnitude, such as the wind, waves, currents); and        monitoring the navigation of streamers.        
In practice, it is aimed to carry out an analysis of sea bed with a minimum number of passages of the vessel in the concerned area. For that purpose, the number of streamers implemented in the acoustic network is substantially increased. This problem of localization of streamers is thus particularly noticeably especially in view of the length of the streamers, which may vary between 6 and 15 kilometers, for example.
Control of the positions of streamers lies in the implementation of navigation control devices (commonly referred as “birds”) installed at regular intervals (every 300 meters for example) along the seismic streamers.
Birds of the prior art are used to control only the depth of the streamers in immersion. Today, the birds are used to control the depth as well as the lateral position of the streamers.
The FIG. 1 shows a configuration of a part of a streamer 13 which comprises a series of sensors (hydrophones) 16, an electro-acoustic transducer 14 (described in more details thereafter) and a bird 10 distributed along its length.
A complete streamer 13 comprises (along its length) a multitude of parts described on FIG. 1, and thus comprises a huge number of sensors (hydrophones) 16 and a series of electro-acoustic transducers 14.
Each bird 10 may be associated with an electro-acoustic transducer 14 and comprises a body 11 equipped with at least one motorized pivoting wings 12 making it possible to steer laterally the streamer 13 and control the immersion depth of the streamer 13.
The control of the birds is made locally or by a master controller situated onboard the vessel.
An acoustic node is commonly known as being a transducer 14 and it's associated electronic. A bird 10 may be associated with an acoustic node 17 to allow this acoustic node to ensure a local control function of the associated streamer 13.
For the horizontal driving, the electro-acoustic transducers 14 allow to estimate the distances between acoustic nodes (named here below “inter-node distances”) placed along two different streamers 13, adjacent or not. More precisely, an electro-acoustic transducer 14 of a first streamer sends several first acoustic sequences and also receives several second acoustic sequences coming from a second electro-acoustic transducer 14 of a second streamer, adjacent or not relative to said first streamer. To estimate an inter-node distance, the data received by a transducer 14 of an acoustic node are then processed locally by an electronic module (not shown on FIG. 1) associated with the transducer 14 or processed by a master controller onboard the vessel.
Transducers 14 are transmitters and receivers of acoustic sequences (i.e. acoustic signals in the form of modulated bits) used to determine distances between adjacent nodes situated on the various streamers, thereby forming a mesh of inter-node distances, in order to know precise lateral positioning of all the streamers.
Transducer here is understood to mean either a single electroacoustic device consisting of a transceiver (emitter/receiver) of acoustic signals, or a combination of a sender device (e.g. a pinger) and a receiver device (e.g. a pression particle sensor (hydrophone) or a motion particle sensor (accelerometer, geophone . . . )).
Usually, each node comprises an electro-acoustic transducer enabling it to behave alternately as a sender node and as a receiver node (for the transmission and the reception, respectively, of acoustic signals). In an alternative embodiment, a first set of nodes act only as sender nodes and a second set of nodes act only as receiver nodes. A third set of nodes can also be used in combination with the first and second sets of nodes. The inter-node distance between two synchronized nodes A and B can be typically estimated on the basis of the following formula:dAB=k×tAB with:    dAB, the inter-node distance separating a sender node (A) from a receiver node (B) of the acoustic signal;    tAB, the propagation duration elapsed between the emission instant and reception instant of the acoustic signal transmitted from the sender node (A) to the receiver node (B);    k, a “measured” or “estimated” value of sound velocity.
As already said, the control of the birds is made locally or by a master controller situated onboard the vessel.
Nowadays, a method widely known for obtaining underwater acoustic sound velocity (or sound velocity for simplification) of acoustic signals transmitted in an acoustic network is the use of sound velocimeters. Indeed, the measurements of sound velocity used by the navigation system are, in general, carried out by means of two sound velocimeters each arranged to two distinct extremities of the network of streamers, thereby providing “measured values” (also called “true values”). By way of example, FIG. 2 shown a network of ten streamers, referred from 20a to 20j, towed by a vessel 21 on which is located a centralized system (not shown) comprising a navigation system and a node manager system. Two velocimeters 22, 23 are positioned on the two outmost streamers 20a and 20j of the set of streamers towed by the vessel, the first one 22 being positioned near the vessel, the second one 23 being positioned at the opposite of the vessel. An estimation of the sound velocity is then carried out by the navigation system at each point corresponding to a position of an acoustic node by observation of the history of real measurements of sound velocity provided by the velocimeters, while taking into account the speed of the vessel.
A drawback of this known method is that, if one of the two velocimeters breaks down, it is necessary to raise the streamer (in which this velocimeter is comprised) out of water, in order to be able to change or repair the defective velocimeter.
Another drawback of this known method is that, to estimate sound velocity of acoustic signals, the navigation system has to suppose that the measured value of sound velocity in a given fixed point is constant over time (in the axis of the streamers). However, in view of the considerable length of the streamers and the low speed of the vessel, there can be several hours elapse between the sound velocity measurement carried out in that given point and the passage of an acoustic node at that same given point. The sound velocity of an acoustic wave in water being, in general, a parameter that rapidly changes particularly with temperature, pressure and salinity of water. Thus, this estimation method provides sound velocity values that are not always reliable. Based on the principle that the average sound velocity of the seawater is equal to 1500 m·s−1, the inventors found that the error in the value of celerity estimated for each acoustic node may frequently reach a few percents, thereby causing an error in propagation duration measurement, and hence in inter-node distance measurement, that may reach the same percentage. It follows that the localization of sensors (hydrophones) distributed along the seismic streamers lacks therefore of precision.
Another drawback of this known method is that the sound velocity measured by a velocimeter to a given point is considered as being constant in the transverse plan to the axis of the streamers (cross-line measurements). For instance, for a network of ten streamers separated each other from 100 meters, the sound velocity is supposed to be constant over the width of network, i.e. 1000 meters. Thereby an approximation, for example by linear or polynomial interpolation, of the sound velocity measured by each velocimeter is carried out in the transverse plan to the axis of the streamers, also making the estimated values of sound velocity unreliable.
In addition, independently of the navigation system, the birds placed along the streamers comprise embedded electronics used for implementing locally a feedback loop (in order to control inter-node distances of the acoustic network). As said before, these inter-node distances are determined as function of the propagation duration of transmitted acoustic signals measured by nodes of the network and an estimated value of sound velocity which is provided, either by the navigation system, or by an operator via the node manager system. The error in this estimated value of sound velocity may therefore cause an error in the feedback of the nodes between themselves.
Another well-known method of estimation of acoustic signal sound velocity consists in measuring in-line propagation duration between two nodes placed on a same streamer and, from knowledge of the in-line distance separating the two nodes, deducing a estimated value of sound velocity. However, in-line propagation duration measurement requires a node structure with an electro-acoustic transducer deported from the streamer (i.e. placed outside the node). Such a known method can not therefore be implemented in the context of network of streamers with transducers integrated into the streamers. Indeed, because of the presence of metallic bodies on some of the streamers, the omnidirectional radiation configuration (or pattern) of transducers is made quasi-omnidirectional or directive, perpendicularly to the axis of the streamers, rendering implementation of the in-line propagation duration measurements impossible.
It should be reminded that the aforesaid problem is described in the particular field of seismic prospecting in a marine environment, but it can be applied in other fields of application.